At least for C129a GPS it's mandatory to check RAIM prior to an instrument approach. It is an almanac look up process.
The C146a GPS units have fault detection, so they may automatically warn of loss of RAIM. But, I have C129a, so I'm not sure.
If you have a RAIM prediction from an authorised source like Air Services or Airways NZ there is no need to do a RAIM check prior to the approach at least In NZ anyway.
A RAIM prediction from Airways is more up to date/accurate than one done in the aircraft.
TSO C129 units will give a RAIM warning.